As is well known, in a CSMA/CD LAN a number of stations are interconnected through a single shared cable (electrical or optical) logically configured as a bus (though possibly physically configured as a bus, tree or star). Data is transmitted between the stations in packets and generally signals are normally only present on the LAN when user data or control data is being transferred (that is, the LAN is idle--no signals present--in the gaps between packets). A station wishing to transmit data first checks that the LAN has been idle long enough for all transmissions to have propagated across the LAN; providing this check shows the LAN to be idle, the station then proceeds to transmit its data. It is, of course, still possible for two stations to start transmission at approximately the same instant and in this case there will be a collision between the two packets being transmitted. Each transmitting station listens out for any such collision and if one does occur, the station erases transmission after having first transmitted a jam sequence that ensures the collision is detected by all the stations concerned. The stations responsible for transmitting the colliding packets will then attempt to retransmit their data after a short, random, delay.
If the LAN is functioning correctly, collisions should only occur within a predetermined time interval following the start of transmission by any particular station. The duration of this time interval is dependent on the longest propagation delay between any pair of stations on the network (or where the network is made up of several segments interconnected by repeaters, between any pair of stations on the relevant network segment), the time taken by a station to launch a packet, and the time taken for a station to detect a collision. In practice, where a LAN is designed in accordance with a particular technical standard, this predetermined time interval will normally be determined with reference to the maximum values of the defining parameters as set down in the technical standard.
In some LAN fault conditions, packet collisions may occur after the initial time interval where such collisions are expected. It is possible to observe such collisions using network analyzers; however, such items of equipment are expensive and complex, requiring much skill to set them up to observe late collisions.
It is an object of the present invention to provide a method and apparatus that facilitate the monitoring of late collisions on CSMA/CD LANs.